// Copyright 2019-2020 Parity Technologies (UK) Ltd.
// This file is part of Substrate.

// Substrate is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// Substrate is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with Substrate.  If not, see <http://www.gnu.org/licenses/>.

use futures::{stream::unfold, FutureExt, Stream, StreamExt};
use futures_timer::Delay;
use linked_hash_set::LinkedHashSet;
use std::time::Duration;
use std::{hash::Hash, num::NonZeroUsize};

/// Creates a stream that returns a new value every `duration`.
pub fn interval(duration: Duration) -> impl Stream<Item = ()> + Unpin {
    unfold((), move |_| Delay::new(duration).map(|_| Some(((), ())))).map(drop)
}

/// Wrapper around `LinkedHashSet` with bounded growth.
///
/// In the limit, for each element inserted the oldest existing element will be removed.
#[derive(Debug, Clone)]
pub struct LruHashSet<T: Hash + Eq> {
    set: LinkedHashSet<T>,
    limit: NonZeroUsize,
}

impl<T: Hash + Eq> LruHashSet<T> {
    /// Create a new `LruHashSet` with the given (exclusive) limit.
    pub fn new(limit: NonZeroUsize) -> Self {
        Self {
            set: LinkedHashSet::new(),
            limit,
        }
    }

    /// Insert element into the set.
    ///
    /// Returns `true` if this is a new element to the set, `false` otherwise.
    /// Maintains the limit of the set by removing the oldest entry if necessary.
    /// Inserting the same element will update its LRU position.
    pub fn insert(&mut self, e: T) -> bool {
        if self.set.insert(e) {
            if self.set.len() == usize::from(self.limit) {
                self.set.pop_front(); // remove oldest entry
            }
            return true;
        }
        false
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn maintains_limit() {
        let three = NonZeroUsize::new(3).unwrap();
        let mut set = LruHashSet::<u8>::new(three);

        // First element.
        assert!(set.insert(1));
        assert_eq!(vec![&1], set.set.iter().collect::<Vec<_>>());

        // Second element.
        assert!(set.insert(2));
        assert_eq!(vec![&1, &2], set.set.iter().collect::<Vec<_>>());

        // Inserting the same element updates its LRU position.
        assert!(!set.insert(1));
        assert_eq!(vec![&2, &1], set.set.iter().collect::<Vec<_>>());

        // We reached the limit. The next element forces the oldest one out.
        assert!(set.insert(3));
        assert_eq!(vec![&1, &3], set.set.iter().collect::<Vec<_>>());
    }
}
